The present invention relates to a method for producing a silicon single crystal wherein size and density of grown-in defect incorporated in the crystal when growing the silicon single crystal by a Czochralski method (hereinafter may be abbreviated to CZ method) are controlled to be a desired value. The present invention also relates to silicon single crystals and silicon wafers produced by the method.
Along with a recent tendency to increase the degree of integration of semiconductor circuits, circuit elements have been becoming finer; thus quality requirements are becoming severer on silicon single crystals produced by CZ method which are used as a substrate. Particularly, there has been required a reduction of defects generated during single crystal growth, such as FPD (Flow Pattern Defect), LSTD (Laser Scattering Tomography Defect), and COP (Crystal Originated Particle), which are called grown-in defect and may cause a degradation in oxide dielectric breakdown voltage characteristics or device characteristics.
Recently, when a silicon single crystal is grown, there has been used a producing method wherein defects incorporated in the crystal during growth of the crystal are suppressed by changing growth conditions or by adjusting temperature distribution within a furnace of a pulling apparatus.
For example, an agglomeration temperature zone of grown-in defects of a silicon single crystal grown by a CZ method is usually in the range of 1150 to 1080xc2x0 C., and size and density of the grown-in defects can be controlled to be desired values by increasing or decreasing a cooling rate of the crystal for passing through the temperature range. When the cooling rate of silicon single crystal in the agglomeration temperature zone of grown-in defects is decreased, an agglomeration of grown-in defects such as COP is promoted, with the result that a silicon single crystal having low crystal defect density can be obtained. Accordingly if thus-obtained silicon single crystal is processed to provide wafers and the wafer is used as a substrate material for integrated circuit device, a density of defects in the wafer surface is remarkably low so that devices having an excellent oxide dielectric breakdown voltage characteristics can be fabricated.
Meanwhile, in the above producing method, although it is possible to keep low density of grown-in defects, there is a drawback that the defect size is in inverse proportion to the defect density and thus the defect size becomes larger as the defect density is reduced (See Japanese Patent Application Laid-open (kokai) No. 10-208987). Along with miniaturization and high integration of integrated circuits, it is said to be undesirable that defects having large size exist in a wafer used as a substrate for integrated circuits, even if the defect density is low.
When the cooling rate of a crystal for passing through the agglomeration temperature zone of grown-in defects is increased by adjusting pulling conditions of the crystal or temperature distribution within a furnace, growth of grown-in defects is suppressed and the defect size itself can be suppressed to be extremely small. However, as the size of defects becomes smaller, the density of defects tends to be higher inversely. In case a number of defects exist on a wafer, a problem in terms of the oxide dielectric breakdown voltage characteristics is caused when the wafer is processed to integrated circuits. Thus, so far the producing method wherein the cooling rate in the agglomeration temperature zone is increased has not been employed so much.
However, recently it was confirmed that even in a silicon wafer in which grown-in defects are generated in the high density, if a size of the defects is very small, the defects can be eliminated by subjecting the wafer to heat treatment. The attention has been riveted to a method in which the defects in the surface of a wafer are suppressed effectively by combining a method for controlling the cooling rate of the crystal in the agglomeration temperature zone of grown-in defects when pulling the silicon single crystal with a heat treatment of the wafer obtained by processing the crystal.
Meanwhile, when nitrogen is added in a silicon single crystal, an agglomeration of defects generated in the crystal during growth of the single crystal is further suppressed. There was recently developed a method in which a size of grown-in defects is kept to be extremely small by controlling properly a time for passing the agglomeration temperature zone in addition to the above method by addition of nitrogen, and then by subjecting wafers processed from the single crystal to heat treatment, the defects in the wafer surface are eliminated (see Japanese Patent Application No. 10-170629). By adopting this method, the silicon single crystal can pass through the agglomeration temperature zone at high speed during the growth, and thereby the defects in the wafer surface can be eliminated without lowering a production efficiency of the crystal. As a result, the oxide dielectric breakdown voltage characteristics are good and the defects having large size do not exist on the wafer, so that the wafer can be used as a substrate material of high quality for integrated circuit. Thus, currently the technique with respect to that method has progressed with great speed.
However, recent test results show some cases where size or density of grown-in defects cannot be regulated evenly and the defect size is dispersed, for example, even though defects can be suppressed by adding nitrogen as impurity and adjusting properly the time for passing of the crystal in a temperature range of 1150-1080xc2x0 C., i.e., an agglomeration temperature zone of grown-in defects, and even though the passing time of the crystal in the above temperature range, i.e. a cooling rate of the crystal in the range of 1150-1080xc2x0 C. is controlled similarly by significantly changing the concentration of added impurity, i.e., nitrogen or by finely adjusting a temperature distribution within a furnace by changing Hot Zone of the pulling apparatus. Besides, even though the wafer obtained by processing is subjected to the heat treatment for eliminating the defects, there are some cases where the defects remain without being eliminated, which causes a problem in production of a wafer having a few defects.
The present invention has been accomplished to solve the above-mentioned problem, and the prime object of the present invention is to provide a silicon single crystal produced by CZ method wherein the dispersion in size and density of grown-in defects is suppressed effectively and the quality is stabilized regardless of the variety of crystals, a silicon wafer and a producing method therefor.
To achieve the above object, the present invention provides a method for producing a silicon single crystal in accordance with a Czochralski method, wherein before producing a crystal having a predetermined kind and concentration of impurity, another silicon single crystal having the same kind and concentration of impurity as the crystal to be produced is grown to thereby determine an agglomeration temperature zone of grown-in defects thereof, and then based on the temperature, growth condition of the crystal to be produced or temperature distribution within a furnace of a pulling apparatus is set such that a cooling rate of the crystal for passing through the agglomeration temperature zone is a desired rate to thereby produce the silicon single crystal.
It has been conventionally said that an agglomeration temperature zone when a silicon single crystal is grown by CZ method is generally in the range of 1150 to 1080xc2x0 C. However, the above range corresponds to a case where any impurity such as nitrogen is not doped in the crystal. It is revealed that in case that impurity of high concentration is added in a growing crystal, the agglomeration temperature zone of grown-in defects varies slightly under the influence of impurities such as nitrogen added for suppression of defects, oxygen supplied from a quartz crucible, boron for providing a characteristic of semiconductor, and the like. In a silicon single crystal manufactured as a product, these impurities of predetermined amount are usually added in accordance with the variety of crystals, and the agglomeration temperature zone also shifts slightly from the range of 1150-1080xc2x0 C. depending on a kind and concentration of the impurities. Furthermore, in the process of investigation and tests regarding the influence of impurities, the inventors of the present invention precisely measured an agglomeration temperature zone wherein any impurity is not added, and found that the agglomeration temperature zone was in the range of 1100-1010xc2x0 C. Hereinafter this temperature range is used unless otherwise indicated.
Accordingly, in order to suppress crystal defects by adding an impurity and besides by utilizing an agglomeration temperature zone of grown-in defects, it needs to grow a single crystal after precise determination of the agglomeration temperature zone which varies depending on a kind or concentration of the impurity in the crystal. Thus in order to grow a silicon single crystal which is processed into product wafers having a stable defect size and density, it is effective to take the following steps; prior to producing the products, performing in advance a growth test of the silicon single crystal including the same impurity as the crystal to be produced, investigating the agglomeration temperature zone of grown-in defects thereof, setting proper condition for growing the single crystal or temperature distribution within a furnace of pulling apparatus, and then producing the silicon single crystal. According to these steps, it is possible to properly keep the cooling rate in the intended agglomeration temperature zone, so that the size and density of grown-in defects can be controlled to be desired values with precision.
In this case, the kind and concentration of the impurity may be at least nitrogen and concentration thereof.
The agglomeration temperature zone varies slightly depending on the kind of impurity and the concentration thereof, and results of test show that, when nitrogen is doped, influence of the impurity is very significant. Therefore, in order to control at least the size and density of grown-in defects in the crystal doped with nitrogen it is desirable to perform a test for determining a change of the agglomeration temperature zone.
According to the present invention, there is provided a method for producing a silicon single crystal in which nitrogen is added as impurity, wherein an agglomeration temperature zone of grown-in defects of the crystal in which nitrogen concentration is in the range of 0.1xc3x971013 to 8.0xc3x971013/cm3 is assumed to shift by xe2x88x9250xc2x0 C. in the high temperature limit and by xe2x88x9220xc2x0 C. in the low temperature limit respectively from an agglomeration temperature zone in a case that nitrogen is not added, and then growth condition of the crystal to be produced or temperature distribution within a furnace of a pulling apparatus is set such that a cooling rate of the crystal for passing through the agglomeration temperature zone is a desired rate to thereby produce the silicon single crystal.
As mentioned above, when nitrogen is added as impurity, as long as the concentration of nitrogen in the silicon single crystal is in the range of 0.1xc3x971013 to 8.0xc3x971013/cm3, if the agglomeration temperature zone of grown-in defects at this time is assumed to be a range wherein the high temperature limit and the low temperature limit are shifted respectively by xe2x88x9250xc2x0 C. and by xe2x88x9220xc2x0 C. from an agglomeration temperature zone of a silicon single crystal without being doped with nitrogen (1100-1010xc2x0 C.), namely, a range of 1050-990xc2x0 C. and then the silicon single crystal is grown, the similar defects suppressing effect can be obtained.
That is, as long as the nitrogen concentration is in the above range, if the high temperature limit and the low temperature limit are assumed to shift respectively by xe2x88x9250xc2x0 C. and by xe2x88x9220xc2x0 C. from the agglomeration temperature zone without nitrogen doping, and then producing condition of the silicon single crystal and temperature distribution within a furnace of a pulling apparatus are set, the error is within allowable range for suppressing grown-in defects and does not significantly affect the size and density distribution of the defects. If such an approximate range is adopted, it is not necessary to perform growth tests for determining the agglomeration temperature zone according to a concentration of impurity in the silicon single crystal before producing products. Thus it becomes possible to produce silicon single crystals effectively, resulting in increase in terms of productivity and yield, and improvement in terms of quality and cost.
In the aforementioned producing method, the average cooling rate of the crystal for passing through the agglomeration temperature zone of grown-in defects is preferably 1.6xc2x0 C./min or more.
As described above, when the impurity is added in a silicon single crystal, if pulling condition or temperature distribution within a furnace of a pulling apparatus is set such that the cooling rate of the crystal in the agglomeration temperature zone is controlled to be rapid, i.e., 1.6xc2x0 C./min or more on the average, the crystal of which the defect size is not dispersed widely can be grown. For example, in case that nitrogen is doped as impurity and its concentration is in the range of 0.1xc3x971013 to 8.0xc3x971013/cm3, the crystal of which the defect size is not dispersed widely and is uniform as 60 nm or less on the average can be grown. If the size of crystal defects is small and is not dispersed widely as mentioned above, even though the defect density of a wafer obtained by processing is high, the defects can be eliminated by subjecting the wafer to heat treatment, so that wafers having high quality can be produced. Besides, even though the defects are not eliminated and still remain in the surface layer of the wafer, the defect size is extremely small, and therefore the defects hardly gives a harmful influence on characteristics of integrated circuit fabricated on the wafer.
Further, the average cooling rate of the crystal for passing through the agglomeration temperature zone of grown-in defects is preferably 1.0xc2x0 C./min or less.
As described above, when the impurity is added in a silicon single crystal, if pulling condition or temperature distribution within a furnace of pulling apparatus is set such that the cooling rate of the crystal in the agglomeration temperature zone is controlled to be slow, i.e., 1.0xc2x0 C./min or less on the average, and then the silicon single crystal is grown, even in case where an impurity etc. is doped therein, it is possible to obtain wafers by processing the silicon single crystal, of which the defect density is extremely low.
According to the present invention, there is provided a silicon single crystal grown in accordance with the above-mentioned method, wherein a density of LSTD before subjecting to heat treatment is 500 number/cm2 or more.
In a crystal having a very high defect density in which a density of LSTD is 500 number/cm2 or more, a size of defect itself is very small as 70 nm or less on the average (number average), even though the density of defects in the crystal is high. Therefore, by subjecting wafers obtained by processing the silicon single crystal to a certain heat treatment, the defects can be eliminated easily. Especially, by keeping the cooling rate of the silicon single crystal in the agglomeration temperature zone to be proper during the growth, as performed in the present invention, as long as the kind and concentration of impurity are almost same, it is possible to make the defect size almost even with a small dispersion. As a result, when the heat treatment for eliminating defects is conducted, further effect of eliminating the defects can be obtained.
According to the present invention, there is provided a silicon wafer produced from the above-mentioned silicon single crystal, wherein the wafer was subjected to heat treatment in an atmosphere of non-oxidizing gas.
This heat treatment for eliminating crystal defects is desirably conducted in non-oxidizing atmosphere such as hydrogen, argon, or mixed gas of both. Condition of heat treatment at this time are not particularly limited as long as the defects are eliminated under that condition. However, the heat treatment approximately at 1200xc2x0 C. for an hour or at 1150xc2x0 C. for two hours is effective. Further, as to selecting the conditions for heat treatment, the time for passing in a desired agglomeration temperature zone is determined to thereby grow a crystal, and then the conditions of heat treatment corresponding to a size of defects generated in the crystal may be selected.
As explained above, if an agglomeration temperature zone of crystal defects which changes depending on a kind and concentration of added impurity is determined in advance, and then based on the temperature, growth conditions of a silicon single crystal or temperature distribution within a furnace of a pulling apparatus is set such that a cooling rate of the crystal for passing through the agglomeration temperature zone is a desired rate and the silicon single crystal is grown, a size and density of the crystal defects, called grown-in defect, which exist in the surface layer of the silicon wafer can be controlled to be desired values without dispersion, and additionally the increase in terms of productivity and yield and the improvement in terms of quality and cost can be achieved.